The essentials
- Only empty orthogonal neighbours are liberties.
- Connected stones of one colour share all their liberties.
- Filling an opposing group’s final liberty captures it.
- Ko prevents an immediate return to the exact previous position.
Atari means one liberty left
A group with exactly one liberty is in atari. If the opponent occupies that final point, the entire group is removed. Atari is both a warning and a tactical opportunity.
Always count the liberties of the complete connected group. A chain may stretch across many intersections and still live or die as one unit.
Captures change liberties immediately
A newly placed stone can appear to have no liberty of its own. If the move fills an opposing group’s last liberty, however, those stones are removed first. Their former intersections become free and the new stone survives.
This order of events explains many surprising edge captures and tight local shapes.
Why suicide is illegal
A move is illegal when the new stone or its group would have no liberty after all captures are resolved. You cannot surround your own stone completely without changing the position.
If the move captures opposing stones and opens new liberties, it is legal. A helpful rules message should explain this reason rather than merely say “invalid move.”
Ko stops an endless loop
Sometimes one stone can be captured and the opponent could immediately capture straight back. Without another rule, both players could repeat the same board forever.
The simple ko rule forbids that immediate repetition. The affected player must move elsewhere first. A serious threat may force an answer, after which returning to the ko can become possible.
Ko is more than a restriction. It can connect one tiny local fight to the strategic value of every move on the board.
